The present invention relates to wireless communications systems and methods and in particular to a signal distribution network for use in these systems.
The basic structure of a conventional wireless network is depicted in FIG. 1. By way of example, the names' given to the various elements of the network shown in FIG. 1 are taken from the Global System for Mobile communications (GSM) standard, which is the most successful and widespread wireless communications system in the world. It must be noted, however, that similar architectures are used with other wireless protocols.
The core network comprises a number of interconnected mobile switching centers (MSC) 1 which have links to a public switched telephone network (PSTN). Each MSC connects to a number of base station controllers (BSC) 2 and each BSC connects to a number of distributed base transceiver stations (BTS) 3. Each BTS is co-located with an antenna 4 which radiates the wireless signals that are generated by the BTSs into free-space. Each BTS provides wireless connectivity to a number of mobile stations (MS) 5 which would commonly be mobile telephones. The coverage area of the wireless network is subdivided into cells 6, each served by a BTS. The link between the BSC and BTS 7 (known as the A-bis interface in GSM), is a baseband digital interface which usually runs on a fiber-optic or microwave radio T1/E1 line.
The wireless signals generated by the BTS comprise a number of channels, each channel being dedicated to a particular MS for the duration of a call. In GSM, the channels are differentiated by a combination of frequency and time. Each frequency, known as a radio carrier, can support up to 8 users on a time division basis. The output of a BTS therefore comprises a number of analog radio carriers, which are radiated by the co-located antenna.
An alternative architecture, as described in U.S. Pat. No. 5,682,256, is depicted in FIG. 2. Here, the BTSs are centralised rather than distributed. In the following text, only the forward link is described (from BTS to MS) for reasons of brevity, although the reverse link (from MS to BTS) is also present. The analog RF outputs from the co-located BTSs are fed into a central RF switch matrix 8. The outputs of the RF switch matrix are connected to a number of optical transceiver units (OTU) 9 which convert the RF signals to optical signals for transmission over optical fiber lines 10. The transmission link is analog in nature and the technique is commonly known as radio-over-fiber. Each antenna site now has a remote antenna unit (RAU) 11 in place of the BTS from FIG. 1. The RAU converts the optical signal from the OTU back into RF form, which is then amplified and radiated from the antenna.
In this architecture, the expensive and complex BTSs are co-located in a benign environment (sometimes known as a BTS hotel), which leads to reduced operational and maintenance costs. Also, the centralization of the radio carriers means that fewer carriers are required for the same grade of service, leading to reduced capital expenditure on BTSs. Furthermore, the RF switch matrix allows capacity to be allocated dynamically, so that fewer still BTSs are needed for situations where capacity demands fluctuate on both a spatial and temporal basis. This type of system can be used in situations where the BTSs belong to more than one network operator, since the switch matrix allows each operator to have their own independent radio plan. A more detailed description of the benefits of this approach can be found in a recent paper by Wake and Beacham, “Radio over fiber networks for mobile communications”, Proc. SPIE, vol. 5466, 2004.
U.S. Pat. No. 5,627,879 discloses a centralized BTS network architecture in which the transmission links between the OTUs and RAUs are digital rather than analog. Digital links have a number of advantages over analog links in applications that require high dynamic range, because analog links suffer from an accumulation of noise and distortion. This architecture combines the radio frequency (RF) analog outputs from the BTSs, performs a frequency down-conversion function and then converts the resulting intermediate frequency (IF) signal to digital form using a fast analog-to-digital converter (ADC). This “digital RF” signal is transmitted using conventional digital optical fiber transmission links to the RAUs, where it is converted back to analog form using a fast digital-to-analog converter (DAC) and then upconverted from IF to RF. This re-constituted RF signal is then amplified, filtered and radiated from the antennas.
U.S. Patent publication U.S. 2001/0036163 takes this basic concept and extends it to describe a multi-operator, multi-protocol centralised BTS system. Here, each BTS output is downconverted and digitized using separate ADCs. The digital outputs of the ADCs are multiplexed at the central hub before transmission and are transported to the RAUs using a common transport system. At the RAU the digital signals are demultiplexed and each protocol or operator has an independent “slice module” which translates the resulting signal to RF.
However there are a number of problems with these approaches, in terms of flexibility of service provision and allocation, resilience to failure and transmission efficiency.